My boy did the exact same. I would come home to step in puddles of drool where he stood by the door panting.
I followed Patricia McDonnell's book to the letter and we made significant progress. I can leave him comfortably now, I dont think he is particularly happy about it but certainly not the stress or anxiety he once felt.
One of the hardest things for me was thinking I wasnt getting anywhere and it was never going to work...but it did. It was a LOT of repetitions of leaving him with a kong toy for short increments to get him there. So it can be done! I think it took about a month to the point where I was ok with leaving him. The training is not linear either, you may take ages to go from 1 minute to 2 minutes but 10mins to 30 mins could be an easy step.
I got a cheap petcam off amazon for £15, I can log on to the app on my phone and check on him. Most of the time he is sleeping on the sofa of course!
Good luck